Dropping your point weight will have the effect of stiffening your shaft.
Not always a bad thing.

Anyway, it may or may not seriously affect your tune.
It depends on where your current arrow's spine value lays within the adjustment capabilites of the rest of your setup.
If you were shooting right near the edge of it before, then something will have to change.

First shoot it and see, but if your paper tune was good before, you'll probably see a right tear with the 100s.
If it is small, try shooting field points vs broadheads for point of impact comparison at several distances.
That will tell you how bad it really is.

If you're not satisfied...
Try moving the rest left (if you're right handed), raise the draw weight, buy new weaker spined arrows, or go back to the 125s.

Don't worry about making any changes to your sights until after you have the arrows flying right.
That should always be the last adjustment, as they have nothing to do with proper flight, and will not help you make good hits when your arrow is fishtailing or porpoising. Penetration is also gained with a good tune since all of the arrow's energy is directed in a straight line driving the point forward.

Good luck.
-Bulz